The present invention relates to the area of ore processing, particularly to the wet processing of phosphate rock.
One of the primary uses of phosphoric acid is in the production of artificial fertilizers. Phosphoric acid is made on an industrial scale by extracting phosphate (expressed as phosphorus pentoxide (P2O5)) from phosphate rock by acidulating the phosphate rock to form a solution of phosphoric acid. In the past, easily mined high-grade phosphate rock deposits provided phosphate without the need for complicated purification processes. With the depletion of such high-grade deposits, lower grade deposits must be employed. However, the presence of a larger amount of impurities, such as metals including iron, in the lower grade rock has necessitated the increased usage of processes for the purification (beneficiation) of the phosphate rock. Such beneficiation processes add to the cost of producing the phosphate and negatively affect the overall P2O5 recovery.
Purification of the phosphate rock is generally desirable in industry, such as in the fertilizer industry, since the absence of impurities means that there is more phosphate present per unit weight of the final acid product. Less of the product is then required leading to a decrease in the cost of transporting it. Furthermore, a higher-grade product leads to a decrease in the amount of handling and reduces the amount of sludge produced during processing. Finally, higher-grade phosphate rock reduces scaling concerns faced by processors.
Fertilizers, for example, may be produced from phosphate solutions by concentrating the phosphoric acid solution obtained from the acidulation of phosphate rock followed by reaction of the solution with anhydrous ammonia to form monoammonium phosphate (MAP) as a wet solid. MAP is then granulated and dried to yield dried granules of fertilizer. Concentration of the phosphoric acid solution may be accomplished by such methods as vacuum evaporation and submerged-combustion direct heating. Superphosphate fertilizers such as normal superphosphate (NSP) and triple superphosphate (TSP) may also be produced from phosphoric acid solutions. For a review of fertilizer production, see Fertilizer Manual, T. P. Hignett ed., (International Fertilizer Development Center, Muscle Shoals, Ala., USA (1985)) pp. 187-202.
The wet processing of phosphate rock generally involves the reaction of ground phosphate rock with an acid such as sulphuric acid or mixtures of different acids. The reactant solution used in such a process is often based on a recycled acid solution already containing phosphoric acid to which more sulphuric or other acid is added. Phosphate dissolves in the acid solution and is present in solution in the form of phosphoric acid. The solution can be isolated from the residue by a variety of methods including filtration, centrifugation and froth flotation. The following reference provides a discussion of the wet process technique: Becker, P., xe2x80x9cPhosphates and Phosphoric Acidxe2x80x9d, Fert. Sc. and Tech. Series, (Marcel Dekker, Inc., N.Y. (1983)) pp. 369-403.
Wet processing has disadvantages. The residue may be slime rather than a crystalline solid making the isolation of the solution more difficult. This can be ameliorated by digesting the phosphate rock for a longer period of time, as in the Prayon process (Slack, A. V., xe2x80x9cPhosphoric Acidxe2x80x9d, Vol. 1, Fert. Sc. and Tech. Series, (Marcel Dekker, Inc., N.Y. (1968) pp. 253-258) thus promoting the growth of larger insoluble solid residue particles, generally gypsum (calcium sulphate) formed from the calcium in the phosphate rock and the sulphate from sulphuric acid. While larger residue particles may be formed in this manner, the longer digestion time results in more of the impurities solubilizing, thus contaminating the product phosphoric acid. The temperature and time of reaction in the Prayon process are responsible for the unwanted solubilization of impurities. Therefore, there is a need for a process that favours the dissolution (and therefore recovery) of phosphate while reducing the amount of impurities recovered with the phosphate and favouring the formation of easily separable residue.
U.S. Pat. No.4,039,624 issued on Aug. 2, 1977 to Hill, discloses a process for producing phosphoric acid from high iron and aluminum content phosphate rocks using nitric acid. This process employs relatively coarse particles of phosphate rock (xcx9c0.5 mm) and the leach time is very long (xcx9c1 hours). Furthermore, this process requires the presence of high levels of iron and aluminum and requires iron in a non-hydrated form (e.g. hematite). There still remains a need for a process that can be generally applied to phosphate rock containing different impurities and that requires less time to effect good separation of the phosphate from the impurities.
U.S. Pat. No. 3,919,395 issued on Nov. 11, 1975 to Hauge discloses a process for extraction of phosphorus compounds from low and high grade phosphate ore using dilute mineral acids whose calcium salts are water soluble. This process employs coarsely ground ore (larger than 100 mesh (150 microns) and requires a neutralization step using ammonia or lime.
In another process, high iron containing phosphate rock is leached with nitric or hydrochloric acid to form a solution of phosphoric acid and a concentrate containing the iron (Forssberg, Eric and Adolfsson, Goran, xe2x80x9cDephosphorization of High Phosphorus Iron Ores by Acid Leaching xe2x80x9d, Erzmetall. 34(6): 316-322 (1981)). This process is focussed on the recovery of iron rather than phosphorus. The process uses relatively coarse particle sizes (in ranges from 75 to 6700 microns) and leaching occurs over a long period of time (xcx9c24 hours). The paper states that sulphuric acid is unsuitable in the process because the formation of calcium sulphate in the concentrate lowers the Fe-content from 61 to 56%. This means that a significant amount of iron is being leached into the acid solution along with phosphate.
U.S. Pat. No. 4,828,811 issued on May 9, 1989 to Derdall et al discloses a process and apparatus for producing phosphoric acid from phosphate ore wherein a slurry of phosphate ore in phosphoric acid is processed in a multi-zone reactor in which coarse solids and xe2x80x9cfinexe2x80x9d solids are processed separately. This patent refers to xe2x80x9cfinexe2x80x9d solids which are typically aboutxe2x80x9465 mesh ( greater than 150 microns).
The effect of particle size on the dissolution of phosphate rock by mixtures of sulphuric acid and phosphoric acid has been studied (Gilbert, Richard L. and Moreno, Edgar C. xe2x80x9cDissolution of Phosphate Rock by Mixtures of Sulfuric and Phosphoric Acidsxe2x80x9d, IandEC Process Design and Development, 4(4): 368-371 (Oct., 1965)). While this study generally shows that reducing phosphate rock to smaller particle sizes favours the dissolution of phosphate, there is no teaching of favourable separation of impurities present in the phosphate rock. There is no indication in this reference that reducing the particle size not only increases the solubilization of phosphate but also selectively increases the solubilization of phosphate in relation to an impurity.
There is provided a process for recovering phosphate from phosphate rock comprising:
(a) leaching finely divided particles of phosphate rock with a protic acid at a temperature and for a time that favours dissolution, into a leachate, of phosphate in relation to an impurity; and,
(b) isolating the leachate.
There is also provided a process for separating phosphate from an impurity in phosphate rock comprising leaching finely divided particles of phosphate rock with a protic acid at a temperature and for a time that favours retention of the impurity in a solid residue in relation to retention of the phosphate in the solid residue.
Phosphate rock includes all naturally occurring mineral deposits containing phosphate as a component. Phosphate deposits can encompass variations and differing compositions within the same source and can have a variety of geological structures and a complex mineral make-up. Apatite and fluorapatite are two variations that may be encountered. Metal ores, such as iron ore, that contain phosphate are also encompassed by the term xe2x80x9cphosphate rockxe2x80x9d.
Phosphate rock includes a number of other constituent elements or impurities that are, ideally, removed or reduced in the recovery of phosphate from phosphate rock. Such impurities include, but are not limited to, main group metals (such as germanium and gallium) and metalloids (such as aluminum and silicon), transition metals (such as iron and vanadium), lanthanide metals, actinide metals, alkali metals (such as sodium) and alkaline earth metals (such as calcium and magnesium). The process of this invention is particularly, but not exclusively, useful for reducing the amount of iron recovered with the phosphate.
Impurities can be present in phosphate rock in a variety of amounts. In the case of iron, high iron content is typically considered to be greater than about 1% by weight based on the total weight of the phosphate rock, and iron contents greater than 5%, greater than 20%, or even greater than 25% are known. The process is particularly suitable for phosphate rock having high iron content since the residue formed is more easily separated because the calcium sulphate particles, to which the iron impurities report, form as large clusters lending themselves to separation by filtration. More costly and complicated separation techniques such as froth flotation and magnetic beneficiation are not required when the residue is formed of large solid particles thus providing economic benefits in the savings of both time and energy. Additionally, the reduced level of impurities results in a better product.
Furthermore, unlike previously discussed U.S. Pat. No. 4,039,624, the present invention works well on both hydrated and non-hydrated forms of iron. It has also been found that satisfactory iron/phosphate separation is achievable by using the present invention on certain mineral forms of iron (e.g. goethite and hematite) that have low magnetic susceptibility for magnetic separation techniques. Conversely, the Prayon process, with its higher temperature and longer reaction time, solubilizes these forms of iron.
In a process of the present invention, a leachate is obtained in which the impurity to phosphorus ratio is markedly smaller than the original impurity to phosphorus ratio found in the phosphate rock. Conversely, the ratio of impurity to phosphorus in the solid residue after leaching is increased in relation to the original rock.
Finely divided phosphate rock provides particularly favourable recovery of phosphate while reducing the amount of impurities recovered with the phosphate. Typically, finely divided particles having a diameter of less than about 200 microns are useful. Particles having diameters of 150 microns or less (xe2x88x92100 Tyler mesh) are preferred. More particularly, particles having a diameter of less than about 75 microns, preferably less than about 60 microns, more preferably less than about 50 microns, yet more preferably less than about 40 microns and even more preferably less than about 38 microns (xe2x88x92400 Tyler mesh) are useful in the process.
Reducing the size of phosphate rock to an appropriate particle size can be done by any convenient method and is generally done prior to leaching the rock with acid. There are a number of well-known prior art methods including grinding and crushing. A variety of such methods are described in Becker, P. xe2x80x9cPhosphates and Phosphoric Acid xe2x80x9d, Fert. Sc. and Tech. Series, (Marcel Dekker, Inc., N.Y. (1983) pp. 195-275). Rod or ball mills, with air classification, are particularly suitable for grinding the phosphate rock for the present invention.
Protic acids are any acids that contribute hydrogen ions (hydronium ions) in aqueous solution. Protic acids include mineral acids and organic acids. Mixtures of protic acids may also be used. Mineral acids or mixtures of mineral acids are preferred. Of the mineral acids, strong acids are preferred, strong acids being those acids that substantially completely dissociate in aqueous solution. Sulphuric acid (H2SO4) and nitric acid (HNO3) are particularly useful with sulphuric acid being more particularly preferred.
Sulphuric acid is particularly preferred since the sulphate from the acid combines with calcium in the phosphate to form calcium sulphate (gypsum) which is insoluble in water and precipitates from solution carrying impurities, particularly iron, with it. Clusters of gypsum are formed which are very filterable. Such easily filterable gypsum particles are formed even when the phosphate rock is leached for a short period of time.
While sulphuric acid is particularly preferred, the use of nitric acid, for example, may also result in a residue that can be separated from the leachate. In the case of nitric acid, calcium nitrate would be formed which can be crystallized and filtered, for example.
The protic acid is preferably used in aqueous diluted form. The amount of sulphuric acid used is typically from about 0.5 to about 5.0 equivalents based on the amount of calcium oxide (CaO) in the phosphate rock. A more preferred range is from about 1.0 to about 3.0 equivalents with a range of about 1.0 to about 2.0 equivalents being yet more preferred and a range of about 1.0 to about 1.5 equivalents being still more preferred.
In the process, the phosphate rock is leached in the presence of the protic acid at a temperature and for a time that favours dissolution of the phosphate in relation to an impurity. It has been found that higher temperature increases the dissolution of both phosphate and iron but increases iron to a greater extent. Shorter time reduces the solubilization of both the phosphate and the iron but, as the leaching time is decreased, the most dramatic reduction in iron dissolution occurs before the occurrence of the most dramatic reduction in phosphate dissolution, particularly at a higher temperature.
It is apparent from these observations that the optimal temperature is a function of the leaching time, and vice-versa. The following algorithms substantially and/or essentially express the relationship between temperature and time as a function of phosphate or iron solubility:
% P=xe2x88x927.025 ln(T)ln(t)+30.796 ln(t)+44.325 ln(T)xe2x88x9297.914 
% Fe=0.001(T)(t)xe2x88x920.029(t)+0.083(T)xe2x88x921.321 
wherein % P is the amount of phosphate solubilized expressed as a percentage of the total phosphate that was present in the phosphate rock originally, % Fe is the amount of iron solubilized expressed as a percentage of the total iron that was present in the phosphate rock originally, T is the temperature in degrees Celsius, and t is the leaching time in seconds. These relationships are particularly pertinent for temperatures from about 40xc2x0 C. to about 70xc2x0 C. and for leaching times from about 60 seconds (1 minute) to about 600 seconds (10 minutes). The percentage of iron not solubilized can then be expressed as:
% Fe not solubilized=100xe2x88x92% Fe
The iron not solubilized would be present in the solid residue.
Generally, the temperature of the leaching step, when performed at atmospheric pressure, may be from about xe2x88x9215xc2x0 C. to about 150xc2x0 C., preferably from about 4xc2x0 C. to about 80xc2x0 C., more preferably from about 10xc2x0 C. to about 70xc2x0 C., even more preferably from about 20xc2x0 C. to about 70xc2x0 C., yet more preferably from about 40xc2x0 C. to about 70xc2x0 C. and even yet more preferably from about 60xc2x0 C. to about 70xc2x0 C. If the leaching step is done under pressure, such as in a pipe reactor, the temperature may even exceed 100xc2x0 C. One skilled in the art will appreciate that the temperature ranges generally used in the process of this invention can be lower than those that are normally employed in the art, such as in the Prayon process. As has been discussed previously, the optimal temperature depends on the leaching time employed.
Generally, the time for the leaching step may be from about 1 second to about 24 hours, preferably from about 5 seconds to about 20 minutes, more preferably from about 30 seconds to about 10 minutes. Times from about 1 minute to about 5 minutes and from about 1 minute to about 3 minutes are particularly preferred in the invention. In embodiments of the invention where the leaching is done under pressure at high temperature, short leaching times on the order of several seconds may provide the good separation of phosphate and impurities that is obtainable from the process of the present invention.
In a particularly preferred embodiment, a process for recovering phosphate from phosphate rock comprises providing finely divided particles of phosphate rock having a diameter of less than about 40 microns, leaching the phosphate rock with sulphuric acid at a temperature from about 40xc2x0 C. to about 70xc2x0 C. for about 1 to about 5 minutes to favour dissolution, into a leachate, of phosphate in relation to an impurity, and, isolating the leachate.
The leaching step can be conducted in any convenient reaction vessel. Examples of such vessels include screw augers, ribbon blenders, paddle mixers and pipe reactors with static mixers that exit directly onto a filtration apparatus.
Once the leaching step is complete, the leachate containing the desirable phosphate must be isolated (separated) from the solid residue containing impurities. Any convenient method of isolation can be used. Well-known methods such as filtration and centrifugation can be used to isolate the leachate. Filtration, particularly vacuum filtration, is preferred since it is easier and more economical.